Composition of matter for cores



Sept. 10, 1940.

in 5400 Mix CROSS KH'IZKtNUI'. LMMINILK A. a. RUDDLE ET AL 2,214,349

COIPOSITION F MATTER FO R CORES Filed Feb. 13, 1939 2 Sheets-Sheet 1Legend- Curve 2 571195. Curve lbs. Curve 4: l2 impacts Curve 5- 6impacts Curve 6- impacfp Curve 7 Cross bend/b9 sfrenqflr Curve I 25 lb:

-- Friabil/fy M0 ldabill'fg Sept. 10, 1940- A. B. RUDDLE ET ALCOIPOSITION OF IATI'ER FOR CORES Filed Feb. 13. 1939 2 Sheets-Sheet 2Cross bendingsfmngfh: Curve u 251:. Curve I? 37 lbs. Curve [3 50 lbs--Friabi/ify: Cwve 14 -I2 impads Curve /5 I 6 imp Curve I6 5 impacfsLegend Weigh Y Bifumen ln blhder (fiodium fiificafe Bifumen) Figure 2Invenfors:

Allan B Rudd/e Earl H. 5p0f5wooa' 5g 'fh eir Afiarneg; {5:42

-Paiented Sept. 10, 1940 PATENT OFFICE oourosrrron or m'rraa. FOB conesAllanEBnddhSanmnciscmandEar-lllenry Spotswood. m Oerrlto, Calif.

Application February 1:, 1939, Serial aste 3 Claims. Thisinventionrelates to molded products and a process for making same, which productshave as their prlncipalingredientflor other similar granular powderysubstances capable of withstanding extremely high temperatures without IIn the art of casting hollow objects, it is customary to prepare a moldwhich represents a negative of the exact outsidecontour of the object tobe cast. Into this mold there is placed a body called the core, havingthe inside contour of the object. After properly closing the mold,containing the core, the metal is then poured into the empty spaceinside the mold through a spout specially provided for the purpose.Depending upon the size and shape of the object to be cast as well asthe metal used, the com- Position of both the mold and the core isvaried widely to-meet varying conditions. All molds and cores areprepared from sand or similar high melting chemically inert granularmaterial, which is held together by a suitable binder. To enable shapingof the sand, the bipdgmust be originally liquid, and upon standingand/or heating to moderately elevated temperatures mustharden,byszappration of a solvent for the send binder b y pol:iifiriaation,etc. Later the complete .molds and cores, when hardened, must be capableof retaining their shape imder the metal-casting conditions for a timeat least sufllcient, until the metal has cooled to a point ofsolidification.

35 Upon fIn-ther cooling. when the solidified hollow object contractsthe core must not be so hard to cause the object to crack, but must besuillciently compressible or friable, to. give way to the contractingmetal.

40 There are a variety of other conditions which the mold and/or thecore must meet. For example in casting very large objects, such as theblock of .a marine Diesel engine, or the housing of a hydroelectricturbine, a common difllculty is the .washing away or errosion of themold or the core by the flowing metals in the vicinity of the intakespout. To prevent this, an extremely resistant g mixture must be used ofvery high sintering temperature, containing a binder which does not 50melt, char or otherwise dislni'Egrate under casting conditions. When theobject is finally cast and has cooled sufllciently to be taken from themold, themold may be broken away by hammer- .ing and chiseling. Thecore, if hard-must likes5 wise be chiseled out, a task which is notdiillcult (our-18s) when dealing with large objects. However, in smallobjects, the chiseling out of the core is neither practical norfeasible, and to make the mass production of small objects such asvalves, grates, small cylinders ,etc=, possible at a reasoni able cost,it must be possible to break out the core by merely hammering the objectfrom the outside. In other words the core must be highly friable andshatterable after casting.

Difllculties which cause frequent failures 01,1 castings are insumcientporosity of the core and evolution of an excessive amount of gas mostlydue to thermal decomposition of the binder. This causes the formation ofbubbles in the casting or may prevent the flow of the molten metalthrough narrow passages in the mold.

Fromthe, above it will be observed that no one sand-binder mixture issuitable for all types of melee of the proper mixture must dependprimarily upon the size of the cast- 20 ing and the temperature of themolten metal.

Our invention is primarily concerned with the preparation of relativelysmall cores, which must possess fair strength after drying or baking atmoderately elevated temperatures, so that they can be handled withoutdanger of breaking, in particular if they have long narrow shapes, orcarry sharp edges, ridges, and the like which may form an important partin the shape of the casting to be produced. Upon casting these coresshould emanate a minimum amount of gas, and their porosity must besuillcient to prevent the building up of substantial gas pressures inthe space to be occupied by the metal inside the mold. Later, when themetal has been cast, the .core 85 should not at once collapse, butshould maintain its shape. But-when hammerirgJLhe cooled cast-{ phalt,'wrches, drying fdttfbils, lead A "oid gllcejlh eige'miit", etbffufitilto-dey @911 Girls used almost ex'clusively for all small cores.

e reasons for this development are somewhat 5 difllcult to explain, forwe have discovered and proven by a large number of experiments thatmixtures of watgghmg andhitmnen, if used with certain precautions andwithin certain limits may provide a binder much superior to linseed oil.

Wehave been able to produce with our mixture, cores which meet all theabove requirements and which materially reduce the number of failures.frequently amounting to as much as 15 or 20% of the total number ofcastings made.

.for shaping and Our preferred core-making composition consistsessentially of sand an a ueous solution of a EELQLEPQW 35 te such ascommerclal waterglass, and a tuniinous material such we. If thecompc'siti'on is too dry molding, an amount of water'or other suitableliquid may be added in am to give it the desired workability ormoldability. While the proper amount of liquid required for goodmoldability will vary with the porosity and coarseness of the sand, wehave found that in general the mixture should contain not more thanabout 20% and normally less than 17% liquid. The amount of water-solublesilicate added in the form of an aqueous solution, but calculated on adry basis, may vary between about 31% to 1.2% by weight of the totalmixture in the absence of bituminous material and up to about 2.6% inthe presence of substantial amounts of bitumen; and the amount ofbituminous material may vary from nothing or a mere trace to about 8times the weight of the added soluble silicate. In order to enable easydistribution of the asphalt throughout the sand mixture, it is preferrmroduce it in the form of an aqueous emgl;

sion.

When the mixture of sand, aqueous silicate solution and asphalt has beenshaped to forms. core, it is baked in an oven to dry and harden it.Waterglass and similar aqueous silicate solutions form upon g hardglassy masses and it is obvious that the cores become stronger withincreasing content of the soluble silicate. n

- the other hand, if the silicate content is too high, the cores retainconsiderable strength after casting of the metal and it may provedifiicult, if not almost impossible, to remove them from the finishedcasting. The addition of bitumen to the core mix has the effect ofgreatly improving the friabllity of the core after casting whilecomparatively little affecting the strength of the core after baking, ifused in amounts not greatly exceeding about 2 times the weight of thesoluble silicate added. When, however, usin quantities of bitumen muchin excess of the above amounts, the strength of the baked core begins todiminish materially. On account of this effect of asphalt on theproperties of waterglass-bound cores, the

- maximum permissible amount of the silicate solution in the sand mixincreases with increasing asphalt content.

In foundry practice it is usual to evaluate the strength of the cores bya crossbending test, approved by the American Foundry Association, inwhich the weight is determined that will break a bar shaped of the coremix and baked. The

bar, 1" x 1" x 8", is placed on supports 6" apart, and a beam, which isloaded with a shot at the rate of 24 lbs. per minute presses down on thecenter of the bar between the supports. 'Ihe weight which breaks the baris registered. It is generally accepted that for average foundry work across bending strength by the above test of 25 lbs. represents theminimum useful limit, al-

though in exceptional cases a'somewhat lower strength may be allowable.

We have found that the strength of cores made' from sand containinggiven amounts of an aqueous solution of a soluble silicate as binder,may vary over wide limits depending upon the baking conditions. Inparticular we have found that CO: and other acidgases, such as $02, H18,etc., have a detrimental influence on the core strength, and that ifcores are baked in the presence of (I): or other acid gases, theultimate strength of the core may be but a small fraction of that whichwould be obtained in the absence of the acid gases under otherwiseidentical conditions. What apparently happens in the presence of acid esis a destruction of the soluble silicateI'the' gaseous acidprecipitating free SiOs which upon drying forms a powder ha no adhesiveproperties, rather than a glass-like continuous mass. It further appearsthat the influence of acid gases is more pronounced if the amount ofsoluble silicate binder is relatively small. It may be that the eflectof acid gases is responsible for the fact that soluble silicates,although previously suggested for core making, have never been used on acommercial scale, since normally core baking ovens are fired directly,the flue gases coming in direct contact with the cores. On account ofthis contact it may have been necessary to use quantities of solublesilicate much in excess of those which will result in good friability,in order to secure sumcient strength of the baked cores, and indeedpublicationsrdisclosing the use of waterglass'for this purpose suggestamounts of the order of 10%. Other factors which affect the strength ofthe core are temperature and time of baking. Suitable temperatures rangefrom about 250 to 600 F. especially good results being obtainable atabout 350500 F. As to the influence'of the time of baking, we have foundthat a maximum strength is usually reached after a given time which mayvary from about minutes to 4 hours. depending upon the size of the core,after which time the strength slowly decreases. Since in practice coresof many different sizes are usually-baked in a single oven an averagebaking time must be used and,consequently small cores may be overbakedand large ones underbaked. Therefore it is desirable to allow a safetyfactor of about 50% above the minimum approved strength of lbs., andwork on the premise that sand mixtures should be used only, capable ofyielding cores having cross bending strengths of not less than 37 lbs.by the American Foundry Association test.

In order to determine the friability or shatterability of the core aftercasting, we have evolved a test comprising heating a 1" cube of the coreto be tested in a substantially closed container in a muflle oven to1750 F. for minutes. Temperature measurements in cores used in actualcasting of iron and steel indicated that this is fairly representativeof the heating to which cores are usually exposed during casting. Theheated cube is allowed to cool and is then placed on a steel plate. Aflat cover of steel,

and a plunger of 100 gr. weight is allowed to drop onto the cover from aheight of exactly 4 cm.

The number of impacts required to shatter the test piece is anindication of its friability. Comparison with foundry tests have proventhat a .friability of 12 impacts represents the upper practical limit,and that for satisfactory friability the number of impacts causingshatterin should be- 6 or less by the above shatter test.

Cores made of sand, silicate solution and asphalt, in order to meet theabove requirements 7 of cross bending strength and shatterability musthave compositions within very narrow limits. These limits are shown inthe attached drawings. which represent composition of graphs, Figure 1showing the composition of the sand mix used for molding the cores, interms of volume per cent cores after casting of the metal.

Referring to Figure 1, on the ordinate of the graph.is plotted thecontent of silicate solution in the sand mix, the content beingexpressed for convenience in volume per cent of a 40 B. sodium silicatesolution contained in the total sand mixture, the average sand used incore making having an apparent density of 1.55. 9n the abscissa isindicated the volume per cent of asphalt emulsion in the binder whichconsists of 40? B. sodium silicate solution-and asphalt emulsion, addedto the dry sand, the emulsion containing 60% bitumen. Curves l-S bendingupward indicate the cross-bending strength by the American FoundryAssociation test hereinbefore described, of cores baked under optimumconditions and in the absence of acidic gases. Curves marked 4-6respectively stand for the shatterabllity of the cores after casting, interms of the described shatter test. Curve I connotes the moldabilitylimit of the sand mix imposed by I foundry practice, and also having ashatterability straight line:

the maximum permissible content of liquid. normally equal to 17% byvolume as stated earlier. It will be observed that the composition ofsand mixes which are suitable for cbre making fall within an area on theplot enclosed between the ordinate and the curves I, I and 4. If any oneof the limitations, for which these curves stand, are transgressed, theresulting core will be unsatisfactory. A preferred area is enclosed bycurves 2 and 5, compositions represented by this area giving coreshaving a fair margin of safety in the cross bending strength, shouldbaking conditions be other than optimum as is the rule in which insureseasy removal of the core from the casting.

The several curves which represent the limits of usefulness andpreferred range respectively, in the composition of the sand mixes canbe expressed in terms of algebraic equations, X being the abscissa and Ythe ordinate, both in the scales hereinbeiore indicated. Curve 4, issubstantially a straight line between the ordinate and the point ofintersection with curve I. Above this point it flattens out to reach aceiling of 1I=7.7. Below this point it has the equation:

Curve 5, although slightly curved in the upper part reaching a ceilingof 6.1, approximates a Curve I is represented by:

Curve 2 is:

2xy-5.1:150y+450=0 and curve 1 equals:

While in preparing the sand mix it is not necessary to use 40 B. alkalimetal silicate solution, nor a 60% is haltegrulsion, it is convenient touse the soluble silicate anmfihalt in the form of aqueous solutions andemulsions, respectively, having these apprqximate concentrations, sincethey are commercially available. It being known that 40 B. sodiumsilicate solution contains about 38% dry sodium silicate, solutions ofdifferentmfim'fiv the same results may readily be substituted to meetthe limitations indicated by the equations. Moreover the asphalt neednot be used as an emulsion but may be mixed with the sand in anyconvenient manner which produces fine distribution of the asphaltthroughout the sand mix. Manifestly. however, by employing emulsionsthis result can most easily be achieved. n the other hand when usingpulverized asphalt there is less likelihood of using too large an amountof liquid which would make the sand mix too wet for good moldability.

In the course'of the baking process, the core is more or less thoroughlydried so that it is composed essentially of sand, glassy water solublesilicate and asphalt. The limits within which the proportions of thesethree components must fall to insure suiiicient cross-bending strengthand shatterability after casting, are shown in Figure 2, which isessentially Figure 1 reduced to the anhydrous state and changed to aweight per cent scale. The several curves which limit the useful andpreferred ranges of compositions of the cores may now be expressed bythe following equations; 3 being the weight percent of bitumen of thebinder consisting essentially of dry sodium silicate and bitumen, and 11being the weight percent of the dry sodium silicate in the mixture ofsand, sodium silicate and asphalt.

Curve l4 (corresponding to curve 4 in Figure 1) is substantially astraight line up to point P (.1:=59') and then flattens out to reach'aceiling of u=2;6. Below P, it follows the equation:

Curve Ii (corresponding to curve 5 in Figure 1) is a straight line belowpoint Q (:c=45) and then flattens out to reach a ceiling of u= 2.05.Below 'Q, it has the equation:

, 9z-400u+40ll=0 Curve ll (corresponding to curve I in Figure 1) is andcurve I! (corresponding to curvef in Figurel) preferably should be below67% as indicated by points R and S, respectively.

Inasmuch as it is desirable to obtain baked cores having a maximumstrength for a minimum number of impacts required to shatter the coreafter casting, it appears that a portion of tively low free al a 1m y,

the useful area in the graphs in which the shatterability curves risehighest over the strength curves indicates an optimum range. As will beseeninFigurelsucharangeexistsoverthearea in which the volume percent ofasphaltemulsionin the binder solution ranges from about 25 to 50%. Thecorresponding range in Figure 2 extends approximately from 30 to 60% byweight of asphalt in the dry binder. optimum relation betweencross-bending strength and shatterability, we prefer to limit ourcomposition to the optimum ratio of asphalt to binder indicated above.

The preparation of the sand mix for making the core is exceedinglysimple. The liquids are simply mixed with the sand in the desiredproportions either by hand or mechanically, and if the mix appears toodry for molding, watermay be added to suit convenience. The order ofadding the alkali metal. silicate solution and asghalt to the y mp ant.ther component may be added first, or both may be added simultaneously;if desired in.the form of a prepared emulsion containing the alkalimetal silicate and bitumen in the preferred proportion.

In the foregoing disclosure we have repeatedly used the term solublealkali metal silicate. this term we mean a water soluble alkali metalsilicate, such as water lass which may contain varying amountm'ilkalimetal hydroxide. As is known, commercial waterglass which is essentiallyan aqueous solution of sodium silicate, comes in various grades ofvarying ratios of NazOtSiO While any of the several commercia grades maybe used, we prefer to employ those having relatively hi h S102contents, 1. e., relak I t Ecause high alkalinity has ammn the skin ofthe workers who shape the cores and necessarily come in direct contactwith sand mix. By selecting a waterglass solution containing S10: andNazO in ratios above about 3.0, we have been able to handle our mixover' extended periods of time with no apparent ill efiect.

45 Furthermore the aqueous alkali metal silicate solution may containsmall quantities of other salts, in particular salts which 'slowly tendto absorb a portion of the free alkali. Salts having this property arefor example the fluosilicates, which are decomposed by free alkali, erey .erating alkali fluorides and precipitating a silicate of ow alkalicontent which is substantially insoluble in water. This absorption offree alkali has a tendency to cause the sand mix to set and to make itless hygroscopic after baking. a property which may be ofconsiderable.importance in moist climates. For example the addition to5% sodium fluosilicate. to a 40 B.

waterglass solutimusea in preparing the 15E may ave s e ect. Ifdesired-small amounts of salts of weak bases and stron acids ay beadded, such as alumm' um sfiifa ammonium To take advantage of this filBy ospolya little or no binding power. For example, we may prepare asoluble silicate piutiomas fol- OwS:

Water :zallrmq 2 Aluminum sulfate ounces -1-10 Alkali fluosilicate do1-10 Waterglass 40 B. (Na:O+3.3SiOz) gals 1-10 The above solution may bemixed with sand and asphalt emulsion as follows:

The bitumen which we employ may be a straightfl er crackedetroleumresidue, or a com tar p roductmd carn or 0 er impurities. Aportion or all of the bitumen may be substituted by other high boilingoranic substances which are chemically subr soluble silicates at normalroom temperatures and under the conditions of baking such as varioustypes of resins, particularly petroleum resins or plasticsTIHEf-i'catingo s e. g. g ycerwars, dex'trins, mew e which mm1m ra e or ecompose underthe conditions of baking,

but carboniz at the temperature of casting. However, bituminousmaterials are preferred because theyevolve the least amount of gasduring casting since they contain an extremely high ratio of carbon tohydrogen or other elements potentially forming gaseous compounds uponheating.

We claim as our invention: 1. The process of making cores with adequatestrength and suflicient friability for foundry practice, which comprisespreparing a mixture of sand, an aqueous solution of a water solublesilicate, and a bitumen, said mixture containing suiiicient normallyliquid components to make it moldable, the amount of the silicate beingfrom 1.2% to 2.6% dry basis, and the amount of the bitumen being notmore than three times the weight of the silicate. and baking saidmixture to substantial dryness in an atmosphere substantially free fromiacidic gases at a temperature below 600 F.

2. The process of making cores with adequate strength and sufficientfriability for foundry practice, which comprises preparing a mixture ofsand, an aqueous solution of sodium silicate, and

a bitumen, the amount of said silicate being from 1.2% to 2.6% drybasis, and the amount of said .bitumen being not more than three timesthe weight of the silicate, and baking said mixture in an atmospheresubstantially free from acidic gases. I

3. The process of making cores with adequate strength and suflicientfriability for foundry practice, which comprises preparing a mixture ofsand, an aqueous solution of a water soluble silicate including asmallquantity of fluosilicate, and a bitumen, said mixture containingsufilcient normally liquid components to make it moldable, but ,not morethan 17 %.thereof,, and baking said mixture in an atmospheresubstantially free from acidic gases at a temperature below 600 F.

, I ALLAN B. RUDDLE.

EARL HENRY SPOTSWOOD.

